1. Field of the Invention
The present invention relates to the field of removing the heat produced by devices on an integrated circuit away from the devices, and more particularly to the use of electroosmotic pumps to circulate cooling fluid to remove the heat from integrated circuits.
2. Discussion of Related Art
The dimensions of devices are shrinking in the integrated circuit (IC) industry while at the same time the number of devices and their respective operations is increasing. All of these factors add to an increase in the heat production of semiconductor devices and the formation of “hot spots”, or areas of intense heat, that develop on an IC during operation. Therefore, effective heat dissipation has become critical in order to further scale down devices and increase their numbers and operations.
Various techniques can typically be used to dissipate the heat generated by the operations of the devices on an IC die. One such technique is the use of a heat sink. As illustrated by the cross sectional view in FIG. 1, a heat spreader 110 is a cap formed of a heat conductive material, such as copper, aluminum, or a ceramic, that is placed above and around an IC die 120 after processing. The heat spreader is not in direct contact with the devices on the IC die because they are pointed down and away from the heat spreader. Also, the heat spreader is not in direct contact with the silicon substrate on which the devices of the IC die are formed because a first thermal interface material (TIM) 130 is placed between the backside of the IC die 110 and the heat spreader 120. A TIM can be a thermally conductive gel, solder, or grease. A heat sink 140 is typically also used for heat dissipation. The heat sink 140 typically has fins 145 that increase the surface area of the sink and thus further dissipate heat. Between the heat spreader 120 and the heat sink 140 a second TIM 150 is placed.
The heat produced by an IC die may exceed the heat dissipation capacity of most passive heat sinks and heat spreaders such as the ones described above. Non-passive heat dissipation devices are being employed to further dissipate heat. One such device is a small cooling fan that can be part of the microelectronic package housing the IC die and the heat sink. Cooling fans can only dissipate a limited amount of heat without becoming unduly large and typically dissipate heat only from the microelectronic package and not from the IC die where the heat is produced. Fans also produce unwanted vibrations.
Another non-passive cooling technique is a water-cooling system. Typically, a water-cooling system transfers heat from the heat sink and heat spreader within the microelectronic package. The hot water formed by this technique is pumped away and continually replaced with cooler water to dissipate heat. The water can run through a series of pipes around the heat sink or through the heat sink. Again, this technique can only dissipate a limited amount of heat before becoming bulky and mainly removes heat from the microelectronic package and not the IC die itself.
To avoid the use of bulky passive and non-passive cooling devices that are limited in their ability to dissipate heat, the industry has turned to the use of cooling devices that can be formed in direct contact with the die within the microelectronic package. One such cooling device is a cooling fluid system formed on the backside of a die or on a thermal management chip that is in direct contact with the backside of a die. An example of such a cooling fluid system is illustrated in FIG. 2. This cooling fluid system utilizes an electroosmotic pump 210 to move water from one side of the substrate 200 to the other. An electroosmotic pump 210 operates through the use of an anode 220 and a cathode 230 formed on either side of a row of trenches 240. The electrical field created between the anode 220 and the cathode 230 will attract ions in a cooling fluid that are present as a result of what is known as an “electrical double layer.” An example of how a double layer forms is when the trenches fabricated in silicon are lined with a thermal oxide. The bonds of the surface of the oxide are terminated by hydrogen ions. When the oxide is in contact with a liquid, one example being water, the hydrogen will release from but stay close to the surface. The hydrogen ions are protons and the liquid in the trenches now contains an excess of positive ions as compared to negative ions. This layer of protons which is close to the trench wall is called the electrical double layer. Hence, the excess positive ions in the cooling fluid will move under the applied electrical field. Because of the viscous drag this creates, the moving excess positive ions pull the surrounding fluid to generate a motion in the bulk fluid. By this mechanism the cooling liquid 250 is pulled though the row of trenches 240 that are the flow-producing region of the pump, and therefore from one side of the substrate to the other to cool the area by continuously pulling in cooling liquid 250 having a lower temperature and pushing out the cooling liquid 250 that has absorbed heat. Typically, the pump generates flow and pressure. After leaving the pump, the fluid moves through channels either etched in the back of the silicon die, in a piece of silicon, or in some other material such as copper which is in contact with the back of the die. The back of the die may be thinned in order to reduce the thermal resistance between the heat generating region of the IC and the back of the die.
An electroosmotic pump 210 will pump the cooling liquid 250 at a faster rate depending on how close the electrodes (anode 220 and cathode 230) are to one another and how close the electrodes 220 and 230 are to the row of trenches 240 that is the flow-producing region of the pump. In the prior art electroosmotic pump 210 of FIG. 2, the electrodes 220 and 230 are bulky wires that cannot be placed in close proximity to the row of trenches 240. For example, the trench length may be on the order of 10 um to 100 um while the electrode wires are on the order of 1 mm apart. The strength of the electric field in the flow producing area of the row of trenches 240 is thus limited by the position of the electrodes 220 and 230 and will not be uniform along the entire depth of the trenches. As a result, the flow rate and cooling ability of the electroosmotic pump is also limited.